The utilization of protective barriers for the prevention of corrosion to the metallic elements of a pipeline wall structure is a common practise.
A coating is normally selected for its ability to withstand the operating temperature of the pipeline and deterioration due to the surrounding conditions such as contact with soil or ground water in which it is to be placed so as to provide a protective layer between the pipe and the surrounding media.
Where the pipeline is positioned in a buried or sub-sea environment, it is also normal for the coating barrier to be supplemented with a cathodic protection system of either an impressed or sacrificial nature. This will act to provide a further level of corrosion protection to the pipeline in the event of minor coating defects. Further, it usually has the additional capability of being able to be enhanced, or having its output increased so as to provide a greater level of protection as the external coating starts to break down or suffer from mechanical damage in the normal course of its life cycle. This provides the means to continue to cathodically protect the external surface of the pipeline from corrosion related damage, even after the effectiveness of the coating barrier diminishes.
It may therefore be stated that in general, the protection of the external surface of a metallic pipeline from corrosion is a relatively straightforward process in terms of its implementation and effective maintenance.
The design and implementation of a corrosion barrier to the internal surface of a metallic pipeline is a much more complex matter however.
It is not usually possible to implement an effective supplementary cathodic protection system to provide protection to internal surfaces of a metallic pipeline. In cases therefore where the media being conducted through the bore of a pipeline is of a particularly high temperature and/or aggressive nature due to its chemical, physical or biological constituents, which will result in a high level of risk from either electrolytic or microbiological corrosion, or erosion, then the utilization of a coating system as described above becomes a non-viable option because of the absolute necessity for a reliable, continuous coating across the field joint region.
The historical tendency within the pipeline industry has therefore been to omit an internal corrosion resistant barrier altogether, when low to medium corrosion rates are anticipated. In this case the metallic pipe wall may be thickened substantially to allow for a calculated corrosion rate to occur over the design life of the pipe structure. An alternative to this, where high corrosion or erosion rates are expected and where practical, it is possible to first complete the pipeline construction and then install a tubular barrier liner of a polymeric or plastic nature in very long lengths, thus overcoming the need for internal field joint coatings.
There is a number of different polymeric and plastic tubular liner types commonly used as internal corrosion barriers and a number of different installation methodologies employed, having been adapted from onshore systems.
In the most simple case, commonly known as loose fit lining, a plastic tube, often manufactured by the extrusion of a polyethylene solid-walled pipe (or another form of plastic), that has an external diameter somewhat less than the internal bore of the host metallic pipeline (typically by 5%), is towed inside the steel pipe bore, using a cable and winch. If operated at a low internal pressure which produces hoop stress in the liner wall less than the yield strength of the polymer from which the pipe was extruded, then the liner will effectively operate as an independent pipe within a pipe. It ill then carry the media independent of the host metallic pipeline, with an annular space existing between the liner and the inner surface of the host pipe wall. Where such a liner is being installed as a means of rehabilitation, the pipeline owner will have to accept the reduction in cross sectional area of the pipeline bore result from the liner insertion, and the production and economic loss.
In many cases however the operating pressure of the pipeline will be greater than the capacity of the plastic liner.
Alternatively, the liner may inflate, until it comes into contact with and is restrained by, the internal surface of the metallic pipe wall. Decades ago this was perceived to be a cost effective manner of effectively producing a tight fit liner against the inner wall of the host pipeline. In later years however, the downside elements of this process have become more widely appreciated. These downside elements include a tendency of the liner to revert back toward its original diameter due to the molecular memory inherent in the polymer from which the liner was extruded, a process known as reversion. This reversion process can happen particularly rapidly where the polymer used for the liner manufacture is permeable to certain of either the gaseous or liquid elements in the media being conducted. The liner reversion process thus gives rise to the re-creation of the annular space, in which the accumulated gaseous and/or liquid elements will accumulate at a pressure that is equal to that of the media in the internal liner bore. This is undesirable inasmuch as 1) the elements accumulated in the annulus may well be in and of themselves corrosive to the metallic pipe wall, 2) any sudden reduction in the operating pressure of the pipeline may cause a large differential between the pressure in the liner bore and annulus, causing a flattening, collapse and even catastrophic failure of the liner.
Another common, more modern method of installing a plastic liner into a metallic host pipeline as a means of corrosion protection is that of tight fit lining. There are various mechanisms that have been created that achieve this and all are aimed at overcoming the deficiencies of the loose fit lining methods which are described above. One method is by the use of a conical reduction die, through which a liner having an external diameter slightly greater than that of the host metallic pipeline is drawn under tension, so that it becomes smaller in diameter than the host pipe bore, into which it is subsequently installed. When the tension on the liner is removed, the liner then quickly reverts back toward its original diameter due to the molecular memory inherent in the polymer from which it is extruded. This outward reversion of diameter or swelling of the liner causes the liner to come into intimate contact with the host pipe wall in such a manner that no annular space exists between the outer surface of the liner tube and the inner surface of the pipe wall. This method is known by various terms such as “Die Drawing”, “Compression Fit Lining”, or “Swagelining”. In the most common applications involving a liner diameter reduction using a die mechanism, a liner comprising an extruded cross section of solid wall polyethylene pipe is used.
Another means of producing a similar tight fit liner result also involves starting with an oversized liner but utilizes a series of rollers or spherical bearings to press inwardly and circumferentially onto the outside surface of the liner in a compressive manner so as to effect the desired liner diameter reduction. This process is commonly known as roller box reduction. With certain (not all) roller box configurations however, the stresses imposed upon the liner during the diameter reduction process are so significant that they exceed the memory retention capabilities of the liner polymer and it is therefore necessary to use a post insertion inflation pressure to ensure a tight fitting liner is produced by the process.
In all of the above examples, the liner section is manufactured of solid, unreinforced plastic which must therefore carry the entire burden of the installation forces as are required to position the liner within the host metallic pipeline. There is a limitation as to how thin the liner wall thickness can be in relation to the liner diameter by virtue of the fact that it is necessary to maintain a high enough liner yield strength so as to enable installation over a long distance, yield strength being a multiple of the polymer yield strength and cross sectional area of the liner wall. For this reason, it is usual for the thickness of the plastic liner produced to considerably exceed that which would be necessary if considered purely from the standpoint of its capabilities to act as an impermeable protective corrosion resistant barrier.
The principal limiting factor in respect of the maximum distance over which any polymeric or plastic type tight-fit liner can be installed within the bore of a host metallic pipeline is therefore always the relationship between the tensile yield strength of the liner and the sum of the pulling force required to overcome the diameter reduction process plus the weight and friction of the liner during the installation process. Liner yield strength is also a factor that is very sensitive to temperature variations as when a plastic polymer becomes warm, its yield strength decreases dramatically, thus in warmer climates, the installation distances of conventional tight fit liners may be reduced considerably. In all cases, a safety factor must also be considered, which will further reduce the distance over which it is permissible to install the liner. Conversely however, at lower temperatures, a plastic liner will tend to become more rigid and stiff making it more difficult to install around even large diameter bends in the host metallic pipeline.
In deep sea applications, where the surrounding temperature is 5° C. or less, the rigidity of the liner is of greater importance. The flexural stresses in the extreme fibres of the liner are a function of wall thickness and so for thick walled liners, the capacity of the liner to resist applied flexural deformation decreases. Such flexural loads include deformation under external hydrostatic pressure, or bending as a result of the liner moving around a bend during installation. A plastic liner that has been manufactured to have a wall thickness adequate enough to provide the capability of being pulled inside a metallic host pipe over a long distance without tensile failure will, by nature, have a limited capability to bend around a corner, or short radius bend due to the applied flexural loads, such as is commonly present on certain types of pipelines, especially those which are positioned sub-sea. As even onshore pipelines are rarely fabricated in a straight line, this ability of the liner to go around bends may be the most significant limitation with respect to how and if such a liner can be installed within a metallic host pipe.
Whilst cross-sectional area, and so wall thickness, increases the liner's tensile capacity for installing into long straight sections, for installing around bends, wall thickness becomes a significant limiting factor due to the reduced flexural capacity. In almost all cases where liners manufactured from existing technologies are used however, the total length of the complete host pipeline structure will exceed the length over which it is possible to install the liner and it is therefore necessary to divide the pipeline into two or more sections, with intermediate terminations and connectors, between which the liners can consequently be successfully installed. In the case of an onshore pipeline this is usually convenient because the location of the terminations points and connectors can be planned so as to be accessible. In the case of a sub-sea pipeline however this fact alone will generally mean that the installation of such a liner is not viable.
Another consideration is that although it may be possible to transport the plastic liner from the point of manufacture and deliver it to the metallic pipeline insertion point in rolls of long lengths in the case of smaller diameters, larger diameter plastic liner pipes cannot be coiled because the diameter of coil that would be necessary so as to avoid the buckling and damaging of the liner would be unmanageable. Normally therefore, larger diameter liner pipes are transported from the factory to the vicinity of the host metallic pipeline in lengths of between twelve and fifteen meters, in a cylindrical, straight form and are welded, or fused together to form a continuous string of liner material of the appropriate length. This welding, or fusion process requires a considerable amount of space to conduct and also a considerable degree of time, equipment, personnel and therefore expense to complete. While the difficulties associated with this welding process can normally be overcome relatively easily in an onshore environment, the transportation, storage, handling and welding together of plastic liner sections in an offshore environment will normally represent such a high degree of difficulty and require such a large amount of space as to render such a project unviable using a conventional plastic liner.
In summary, the characteristics of plastic liners that are currently available for the purpose of providing a corrosion resistant barrier to the internal surface of a metallic pipeline have the following characteristics and issues:                the wall thickness is usually determined by the requirement for tensile strength more than other performance abilities;        there is a limit to the straight line distance over which these liners can be installed;        the distance over which installation is possible may be dramatically reduced when there are bends in the pipeline;        it is not normally viable to install a liner from one end of a pipeline to the other in a single length without the need for intermediate connections.        liners cannot be installed around short radius bends such as are commonly found in sub-sea pipelines, due to the limitations in flexural strength and a tendency to buckle and kink.        
The above factors indicate that in almost all circumstances, it is impossible to use currently available plastic liners and lining techniques for the internal corrosion protection of sub-sea pipelines.